Existing hot melt adhesive dispensers operate at relatively high temperatures, such as above about 250° F. Present dispenser configurations have high temperature surfaces exposed to personnel. Considerable measures are taken to guard or insulate the dispensing equipment from nearby personnel. However, this also reduces the ease with which the equipment may be serviced by such personnel.
Many hot melt dispensers include a heated manifold for supplying hot liquid adhesive to one or more valve modules. Very often, these manifolds are heated by cartridge heaters or other heating elements contained within the manifold. The manifold may therefore contain high tolerance bores for receiving the heaters. Air gaps can exist between the heaters and the manifold resulting in localized hot spots or overheating. Over time, these hot spots will cause heater failure. In some cases, it may also be difficult to obtain highly uniform heating of a manifold through the use of internal heaters. For example, small manifolds or irregularly-shaped manifolds may not easily permit the use of cartridge heaters or cast-in-place heaters.
Present methods of supplying liquid hot melt adhesive can also result in adhesive stagnation and air pocketing. This contributes to char formation and related overheating problems which then adversely affect dispenser performance. Also, the typical circular cross sectional flow area of liquid supply passages is an inefficient heat transfer configuration. Many manifolds are also constructed of cast metal thus leading to lower strength threads and difficulty in accommodating a liquid filter.
Another problem arising when dispensing viscous liquids, such as hot melt or room temperature adhesive, relates to the formation of tailing, stringing or drooling of adhesive upon liquid cut-off. The inertial effects of fluid flow may prolong adhesive cut-off, therefore resulting in these undesirable effects. In a traditional valve arrangement, liquid adhesive flows parallel to a valve stem into the valve seat area. When the end of the valve stem is lifted from the seat, the flow path is relatively straight. As the valve stem approaches the seat, the liquid inertia combines with the decreasing flow area between the valve stem and the seat edge thereby resulting in increased liquid flow velocities. These increased velocities can lead to stringing, tailing or drooling of adhesive after cut-off. When dispensing hot melt adhesives, the same cut-off problems can arise if the adhesive is not maintained at the proper set point temperature in the nozzle.
It would therefore be desirable to provide dispensing apparatus for dispensing liquid hot melt or room temperature adhesive and overcoming problems in the art such as those mentioned above.